The present invention relates to a differential infrared detector which detects the presence of a person by detecting a difference between the surface temperature of a human body and the ambient temperature.
Generally, a differential infrared detector detects a difference between the surface temperature of a human body and the ambient temperature, and generates a signal which corresponds to the difference. As shown in FIG. 11, the temperature difference decreases as the ambient temperature approaches the surface temperature of a human body (e.g. 37xc2x0 C.). As for the ambient temperature, it varies considerably from season to season, and, even in one season, daytime temperature and nighttime temperature may change greatly. Hence, the differential infrared detector is desired to take its installation environment into consideration. To deal with a narrow difference between the surface temperature of a human body and the ambient temperature, conventional techniques have attempted to increase the detection sensitivity. Nevertheless, a conventional detector with an increased detection sensitivity detects even a negligible change of the radiation energy in the detection area, which results in improper operations. In order to solve this defect, Japanese Examined Patent Publication No. S61-18236 discloses a sensitivity-compensatory detection apparatus. With detecting the ambient temperature, this detection apparatus increases the detection sensitivity if the ambient temperature falls in a high temperature range, whereas it decreases the detection sensitivity if the ambient temperature is in a low temperature range.
Regarding the detection sensitivity problem which results from a change of the difference between the ambient temperature and the human body temperature, another solution is disclosed in U.S. Pat. No. 5,629,676. This disclosure seeks to apply an ideal amplifier gain to a PIR (passive infrared) detector, particularly when the ambient temperature exceeds the human body temperature. This technology employs a microprocessor. Referring to a circuit diagram of the PIR intrusion detector in FIG. 7, an output from a PIR sensing element 12 biased by a resistor 15 or from a thermopile or pyroelectric device is fed, as an amplified contrast signal 16, to an A/D port 18 of a microprocessor 17. In addition, an ambient temperature signal 21 from a thermistor 22 is fed to an A/D port 20 of the microprocessor 17. Thus, these signals implement a computer program. The computer program recognizes the ambient temperature based on the output from the thermistor 22, and compares the amplified contrast signal 16 with a variable threshold value dependent on the value of the ambient temperature. If the comparison result indicates the presence of a person, the computer program generates an alarm signal. In this technology, a series of signal processing operations are controlled by a computer.
The present invention intends to realize a differential infrared detector whose detection sensitivity does not deteriorate even when the difference between the surface temperature of a human body and the ambient temperature is small. And yet, this detector is obtainable at a low production cost and thus highly economical.
In order to achieve this object, an improved differential infrared detector of the present invention is of the type having: a detection element which detects infrared radiation in a predetermined detection area, converts a radiation energy thereof into an electric signal, and outputs the signal; and a discrimination circuit which calculates, based on the output signal, a variation amount of the radiation energy which varies on entry of a person into the detection area, the discrimination circuit comparing the variation amount with a predetermined level and, if the variation amount is equal to or greater than the predetermined level, generating a human body detection signal. This differential infrared detector is characterized in comprising: an ambient temperature detection circuit which generates a voltage, based on an ambient temperature in the detection area; and a sensitivity correction circuit which receives the voltage from the ambient temperature detection circuit and discriminates, based on the received voltage, whether the ambient temperature is within a predetermined temperature range, the sensitivity correction circuit being arranged to increase detection sensitivity of the output signal produced by the detection element, if the ambient temperature is judged to be within the predetermined temperature range.
Owing to this arrangement, the ambient temperature detection circuit generates a voltage which changes with ambient temperature. Therefore, the ambient temperature can be specified by recognizing the voltage. Accordingly, the sensitivity correction circuit can recognize the ambient temperature based on the voltage, and discriminate whether to raise or lower the detection sensitivity of the output signal which is produced by the detection element.
To be specific about this arrangement, the ambient temperature detection circuit comprises a thermistor and two series-connected resistors one of which is connected with the thermistor. As for the sensitivity correction circuit, it may comprise a logic circuit which receives input voltages composed of a voltage between the two resistors and a voltage between the thermistor and the resistor connected with the thermistor. Based on the input voltages, the logic circuit discriminates whether the ambient temperature is within a predetermined temperature range. If the ambient temperature is judged to be within the predetermined temperature range, the logic circuit generates a sensitivity switching signal for increasing an amplifier gain.
Further, the sensitivity correction circuit may comprise an amplifier circuit which is connected to a later stage of the logic circuit and which has a plurality of resistors provided on an input side of the amplifier circuit. In this case, an increase rate of the amplifier gain is determined by a resistance ratio of these resistors.
Alternatively, this logic circuit may generate a sensitivity switching signal for lowering a trigger level, if the ambient temperature is within the predetermined temperature range.
Regarding this arrangement, the sensitivity correction circuit may comprise a comparator which is connected to a later stage of the logic circuit and which has a plurality of resistors provided on an input side of the comparator. In this case, a lowering rate of the trigger level is determined by a resistance ratio of these resistors.
The logic circuit applied in the above arrangements is constituted with an Exclusive-OR circuit (hereinafter, xe2x80x9cXOR circuitxe2x80x9d) or an equivalent circuit of the XOR circuit.
Preferably, a resistor is connected in between an output stage of the logic circuit and a previous stage of the amplifier circuit, and also connected in series with a grounded capacitor. In this arrangement, a voltage between the resistor and the capacitor is supplied, as a sensitivity switching signal, to the amplifier circuit.
As a result, the sensitivity switching signal supplied to the amplifier circuit has a waveform which rises and falls gently, so that generation of noise is suppressed when the sensitivity is switched.